In the chick ciliary ganglion, preganglionic terminals maintain cholinergic synapses on the choroid neurons and both cholinergic and electrical synapses on the ciliary neurons. The preganglionic terminals also contain enkephalinand substance P-like immunoreactivity, suggesting that transmission through the ganglion is more complicated than is indicated by the known synaptic connections.We report here that embryonic chick ciliary ganglion neurons also have yaminobutyric acid (GABA) receptors and that GABA applied to the ganglion can block transmission elicited by preganglionic stimulation.Studies on the neurons in cell culture indicate that the GABA response is mediated by GABAn receptors: GABA activates a Cl-conductance, and the response can be mimicked by muscimol and blocked by bicuculline or picrotoxin. The GABA receptors are regulated independently from acetylcholine (ACh) receptors on the neurons since the levels of ACh and GABA sensitivity are influenced differently by culture age and by chronic exposure to GABA or elevated K+ concentrations.Application of GABA to intact ciliary ganglia increases the membrane conductance of ganglionic neurons (as in culture), reduces to subthreshold the amplitude of excitatory postsynaptic potentials in the neurons elicited by preganglionic stimulation and completely blocks transmission through the ganglion.A native source of ligand for the receptors in viva has yet to be identified.Autonomic ganglia have been likened to simple relay stations in which little processing or modification of the signal occurs as it is transmitted from preganglionic to postganglionic neurons. This once seemed particularly true for the chick ciliary ganglion where no interneurons or synapses between ganglionic neurons have been identified in vivo. The ganglion contains two populations of neurons: choroid neurons that innervate smooth muscle in the choroid layer, and ciliary neurons that innervate striated muscle in the iris and ciliary body. Both populations of neurons are in turn innervated by preganglionic neurons located in the Edinger-Westphal nucleus of
Chick ciliary ganglion neurons have nicotinic acetylcholine receptors (AChRs) that mediate synaptic transmission through the ganglion. A soluble component of about 50 kDa from embryonic eye tissue, the synaptic target of the ganglion, increases the development of ACh sensitivity by the neurons 10-fold over a 1-week period in culture. The increased sensitivity does not arise from a change in agonist affinity or esterase activity. Both the basal ACh response obtained in the absence of the 50-kDa component and the elevated responses obtained with it can be inhibited by neuronal bungarotoxin (nBgt) but not by alpha-bungarotoxin (alpha Bgt). Increases of less than twofold are observed for the binding of anti-AChR monoclonal antibody 35 (mAb 35), nBgt, and alpha Bgt to the neurons under these conditions. Extract fractions containing the 50-kDa component also enable the neurons to enhance their ACh responses through a cAMP-dependent mechanism. Either the 50-kDa fraction induces the appearance of a new type of AChR regulated by cAMP, or it alters the function of existing AChRs. The 50-kDa fraction produces no change in neuronal growth but can increase GABA responses sixfold, indicating that its effects are not confined to AChRs. It is not clear whether a single molecular species is responsible for the diverse regulatory effects or whether several types of active components are present in the fraction. The component which enhances ACh sensitivity is trypsin-sensitive and heat-labile, as expected for a protein. The component may be widely distributed since the 50-kDa fraction from a number of tissues can increase the ACh response. The fraction from eye tissue, however, has a specific activity 5-10 times greater than that of the liver fraction. A wide distribution would suggest multiple targets and roles for the component during development.
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